1. Field of the Invention
This invention relates to an expansion engine operating on the Brayton cycle to produce refrigeration at cryogenic temperatures.
2. Background Information
A system that operates on the Brayton cycle to produce refrigeration consists of a compressor that supplies gas at a discharge pressure to a counterflow heat exchanger, which admits gas to an expansion space through an inlet valve, expands the gas adiabatically, exhausts the expanded gas (which is colder) through in outlet valve, circulates the cold gas through a load being cooled, then returns the gas through the counterflow heat exchanger to the compressor. U.S. Pat. No. 2,607,322 by S. C. Collins, a pioneer in this field, has a description of the design of an early expansion engine that has been widely used to liquefy helium. The expansion piston is driven in a reciprocating motion by a crank mechanism connected to a fly wheel and generator/motor. The intake valve is opened with the piston at the bottom of the stroke (minimum cold volume) and high pressure gas drives the piston up which causes the fly wheel speed to increase and drive the generator. The intake valve is closed before the piston reaches the top and the gas in the expansion space drops in pressure and temperature. At the top of the stroke the outlet valve opens and gas flows out as the piston is pushed down, driven by the fly wheel as it slows down. Depending on the size of the fly wheel it may continue to drive the generator/motor to output power or it may draw power as it acts as a motor. The inlet and outlet valves are typically driven by cams connected to the fly wheel as shown in U.S. Pat. No. 3,438,220 to S. C. Collins. This patent describes a mechanism, which is different from the earlier patent, that couples the piston to the fly wheel, one which does not put lateral forces on the seals at the warm end of the piston. U.S. Pat. No. 5,355,679 to J. G. Pierce describes an alternate design of the inlet and outlet valves which are similar to the '220 valves in being cam driven and having seals at room temperature. U.S. Pat. No. 5,092,131 to H. Hattori et al describes a Scotch Yoke drive mechanism and cold inlet and outlet valves that are actuated by the reciprocating piston. All of these engines have atmospheric air acting on the warm end of the piston and have been designed primarily to liquefy helium, hydrogen and air. Return gas is near atmospheric pressure and supply pressure is approximately 10 to 15 atmospheres. Compressor input power is typically in the range of 15 to 50 kW. Lower power refrigerators typically operate on the GM, pulse tube, or Stirling cycles. Higher power refrigerators typically operate on the Brayton or Claude cycles using turbo-expanders. U.S. Pat. No. 3,045,436, by W. E. Gifford and H. O. McMahon describes the GM cycle. The lower power refrigerators use regenerator heat exchanges in which the gas flows back and forth through a packed bed, gas never leaving the cold end of the expander. This is in contrast to the Brayton cycle refrigerators that can distribute cold gas to a remote load.
The amount of energy that is recovered by the generator/motor in the '220 Collins type engine is small relative to the compressor power input so mechanical simplicity is often more important than efficiency in many applications. U.S. Pat. No. 6,202,421 by J. F. Maguire et al describes an engine that eliminates the fly wheel and generator/motor by using a hydraulic drive mechanism for the piston. The inlet valve is actuated by a solenoid and the outlet valve is actuated by a solenoid/pneumatic combination. The motivation for the hydraulically driven engine is to provide a small and light engine that can be removably connected to a superconducting magnet to cool it down. The claims cover the removable connection.
U.S. Pat. No. 6,205,791 by J. L. Smith describes an expansion engine that has a free floating piston with working gas (helium) around the piston. Gas pressure above the piston, the warm end, is controlled by valves connected to two buffer volumes, one at a pressure that is at about 75% of the difference between high and low pressure, and the other at about 25% of the pressure difference. Electrically activated inlet, outlet, and buffer valves are timed to open and close so that the piston is driven up and down with a small pressure difference above and below the piston, so very little gas flows through the small clearance between the piston and cylinder. A position sensor in the piston provides a signal that is used to control the timing of opening and closing the four valves. If one thinks of a pulse tube as replacing a solid piston with a gas piston then the same “two buffer volume control” is seen in U.S. Pat. No. 5,481,878 by Zhu Shaowei. FIG. 3 of the '878 Shaowei patent shows the timing of opening and closing the four control valves and FIG. 3 of the '791 Smith patent shows the favorable P-V diagram that can be achieved by good timing of the relationship between piston position and opening and closing of the control valves. The area of the P-V diagram is the work that is produced, and maximum efficiency is achieved by minimizing the amount of gas that is drawn into the expansion space between points 1 and 3 of the '791 FIG. 3 diagram relative to the P-V work, (which equals the refrigeration produced).
The timing of opening and closing the inlet and outlet valves relative to the position of the piston is important to achieve good efficiency. Most of the engines that have been built for liquefying helium have used cam actuated valves similar to those of the '220 Collins patent. The '791 Smith, and '421 Maguire patents show electrically actuated valves. Other mechanisms include a rotary valve on the end of a Scotch Yoke drive shaft as shown in U.S. Pat. No. 5,361,588 by H. Asami et al and a shuttle valve actuated by the piston drive shaft as shown in U.S. Pat. No. 4,372,128 by Sarcia. An example of the multi-ported rotary valve similar to the ones that are described in the present invention is found in U.S. patent application 2007/0119188 by M. Xu et al. U.S. Pat. No. 6,256,997 by R. C. Longsworth describes the use of “O” rings to reduce the vibration associated with the pneumatically actuated piston impacting at the ends of the stroke. This can be applied to the present invention.
It is an object of the present invention to achieve good efficiency with a relatively light weight, compact, and reliable engine. Another objective is to have an engine that can be adapted to cooling a large mass from room temperature to a cryogenic temperature while fully using the compressor output, or optimized to produce refrigeration over a small range of cryogenic temperatures. A final objective is to have a Brayton cycle engine in the same size range as present GM cycle refrigerators so that the cold gas flow from the engine can be used to cool distributed loads.